1. Field of the Invention
The present invention relates to an optical information recording medium, an optical information recording apparatus and an optical information recording method for recording information as hologram.
2. Description of the Related Art
High-density stacked-volumetric optical disks using holography (hereinafter, “holographic optical disks”) and record-playback apparatus for the holographic optical disks have been actively developed for practical use. The holographic optical disks record information by interfering an information beam that includes an image to be recorded with a reference beam for recording the image in a photosensitive material, and records a two-dimensional image resulting from digital encoding by a spatial light modulator such as a liquid crystal device and a digital micromirror device. The information is three-dimensional record that can be recorded in the longitudinal direction of an information recording layer. The information can be multiplexed at an identical location or overlapping locations of the information recording layers. This allows much more information to be recorded than conventional optical recording method of recording information in a plane such as high definition digital versatile disk (HD DVD) standard and Blu-ray standard. Moreover, because the information can be read in the unit of two-dimensional image, the holographic optical disk is advantageous in that the information can be transferred at a high speed.
Among various technologies developed for the record-playback apparatus for the holographic optical disk, collinear holographic recording method in which the information beam and the reference beam are aligned on the same axis is focused on as a replacement of the optical disk record-playback apparatus compliant with the HD DVD or the Blu-ray.
Technologies of the collinear holographic recording are disclosed in “Advanced Collinear Holography” (Optical Review, Vol. 12, No. 2, 90-92 (2005)), “A Novel Collinear Optical Setup for Holographic Data Storage System” (Proceedings of SPIE of Optical Data Storage 2004, pp. 297-303 (2004)), JP-A 2004-265472 (KOKAI), and the like. Based on the collinear holographic recording technology, the spatial light modulator produces the information beam and the reference beam as a record-playback laser by modulating the light intensity of a green or blue-violet laser beam, and an objective lens focuses the information beam and the reference beam on the information recording layer of the optical disk. The information beam and the reference beam are overlapped in the information recording layer to produce an interference fringe pattern, and the interference fringe pattern is fixed in the information recording layer, whereby the information is recorded as a hologram.
The collinear holographic recording method uses a shift multiplexing method of recording the information while the holographic optical disk rotates, namely while shifting locations on the holographic optical disk. The information on the holographic optical disk is played back by focusing only the reference beam for playing back on the information recording layer using the objective lens, accepting the diffracted light from the interference fringe pattern in the information recording layer by a solid-state image sensor using a complementary metal oxide semiconductor (CMOS) or charge-coupled device (CCD), and decoding the accepted two-dimensional image.
A focusing servo and a tracking servo on the holographic optical disk use a red laser beam whose wavelength is different from that of the record-playback laser (a green or blue-violet laser beam), and the servo control is performed in the same manner as with the existing record-playback method such as the HD DVD and the Blu-ray.
More specifically, when the conventional technology is used, the record-playback apparatus for the holographic optical disk cannot rotate the disk at a high speed at the time of recording, thereby an overhead time is required for shifting from the recording operation to the playing back operation, and seek time cannot be shortened.
In other words, to write refractive-index modulation in the information recording layer using the information beam and the reference beam, a certain exposure time is required. When the optical disk rotates at the high speed to record the information, an exposure position moves, and therefore a sharp interference-fringe pattern cannot be formed. The displacement during the exposure is a very serious problem in the system architecture because the holographic recording includes writing fine refractive-index modulation using the photosensitive material that has a high spatial resolution.
When the optical disc rotates at a low speed to prevent the displacement during the exposure, the record-playback apparatus cannot change from the recording operation to the playing back operation at the high speed.
To solve the problem, JP-A 2003-85768 (KOKAI) discloses a technology using a following up mechanism that makes an irradiating light follow the rotation of the optical disk. The following up mechanism enables the record-playback apparatus to irradiate the information beam and the reference beam while tracking the disk rotation so that the relative velocity of the beams to the optical disk is zero during the exposure.
According to the technology, a record-playback laser beam and a servo laser beam are focused on the same plane, and the optical disk includes pits for address servo, information recording area, and lockup pits for following up intermittently on an identical track. By the servo laser beam irradiating so as to follow the lockup pit, the record-playback apparatus detects an amount of disk travel and moves the record-playback laser beam to record the hologram based on the detected amount.
However, according to the technology disclosed in JP-A 2003-85768 (KOKAI), a position of the source of the following up laser beam is displaced sequentially from an optical axis of a collimating lens according to the recording position. In other words, the light source itself is mechanically driven, and therefore the light source needs to be mounted with a high accuracy. As a result, the following up is not accurate enough to put the record-playback apparatus to practical use.
Moreover, because the relative velocity of the recording beam to the optical disk becomes zero during the following up, it is difficult to perform an accurate tracking servo control. It is therefore difficult to perform an accurate positioning of the recording beam to the target recording position.